Mesodermal and Somite Development

While the notochord is forming (see neuroembryology), the mesoderm is growing on either side of the midline (as the primitive streak recedes)- forming 3 columnar structures:paraxial, intermediate and lateral plate mesoderm.

The paraxial mesoderm (like those that have formed the notochord), originally come from epiblast cells migrating through the primitive node/cranial portion of primitive streak.  It forms a pair of cylindrical structures that are adjacent to the neural tube and notochord.

• At around day 24/25 the paraxial mesoderm begins to segment (beginning at the cranial end, working caudally) into somites.  By day 32, around 35-37 somite pairs are present.  Somites will go on to become the axial skeleton (sclerotome), striated muscle (myotome) and subcutaneous tissue and skin (dermatome).  The arrangement and order of the somites will determine that of the spine, spinal nerves, trunk and chest (ribs etc).

Early Embryology

First steps

After fertilisation in the ampulla of the fallopian tubes, division of the diploid zygote occurs to form the morula.  The morula is a cluster of cells than cannot grow/multiply any further due to the restriction imposed by the, still present, zona pellucida.  It is formed by around 96 hours (day 4), by which time it has reached the uterine part of the fallopian tube.  This movement is brought about by tubal cilia and smooth muscle contractions.

The cells then organise themselves (engendering) to form the blastocyst.  The morular cells in contact with the zona pellucida join together, compact and flatten to become an epithelial layer (trophoblasts); the few inner cells develop into the inner cell mass (embryoblasts), which concentrate at one pole of the blastocyst, creating a cavity in its centre.  By the end of day 5, the blastocyst has fully formed and ‘hatches’ from the zona pellucida by a series of enzymatic reactions.

Implantation

Once hatched the embryo comes into contact with the uterine mucosa.  Adhesion can only occur if the endometrium is in the secretory phase of the menstrual cycle, particularly around 6-10 days after the LH surge and ovulation (the implantation window).  Adhesion occurs between microvilli on the outer surface of trophoblasts and epithelial cells of the uterus.  Syncytiotrophoblasts then burrow through the endometrial epithelium, also by releasing a series of enzymes that break it down.

This process continues, and eventually the entire embryo lies within the endometrium.  During week 2, vacuoles (lacunae) form in the syncytiotrophoblast zone.  Maternal blood vessels are invaded and fill these vacuoles with blood (the blastocyst can then begin to receive nutrients from the mother).

The Bilaminar Cell Layer

The embryoblasts, prior to implantation, are arranged into two layers: the Hypoblast (ventrally) and the Epiblast (dorsally).  During implantation, these layers form the umbilical vesicle and amniotic cavity (respectively).  The point where the two cell layers lie on each other is the embryonic plate (this is oval shaped).

Gastrulation

At the beginning of the 3rd week, epiblast cells begin to proliferate to form the primitive streak– which is a linear structure that lies along the rostro-caudal axis (the rostro-caudal (or cranio-caudal) axis defines the head-tail axis).  This structure effectively becomes raised around its border (at its caudal end to become the primitive node) and deepens in the centre to become the primitive groove (at its caudal end the primitive pit– which will go on to become the head/brain etc.)

This is important because the groove provides an entry point for the epiblast cells to migrate between the layers and into the hypoblastic layer to form the trigeminal layer, a process known as gastrulation (occurs around 17 days).

Transition from bilaminar to trilaminar:

• Epiblast becomes:
  • ectoderm(dorsally)
  • mesoderm (middle)
  • endoderm (ventrally)- NB just the intraembryonic compartment
• Hypoblast becomes:
  • extraembryonic endoderm i.e. umbilical vesicle and allantois (umbilical cord)

These three cell types make up the all the tissues of the human body.